This invention relates to an optical fibre arrangement, optical fibre lasers and optical fibre amplifiers.
There is a demand for optical amplifiers that can output powers of 1 W or greater, can amplify many wavelength channels simultaneously with low cross-talk and low inter-channel interference, and can do so with high reliability and low cost per wavelength channel.
In many applications such as dense wavelength division multiplexing (WDM) transmission systems and satellite communications, optical amplifiers and transmitters, optically pumped by, e.g., laser diodes, should not only be capable of handling relatively high power but also be protected against failure of pump sources.
Conventional optical amplifiers use single-mode optical fibre whose core is doped with one or more rare-earth ions such as Erbium. Such amplifiers provide limited power output that is insufficient for multi-channel WDM transmission systems. In addition, conventional amplifiers are prone to the failure of pump sources, requiring several pump sources to be contained with the amplifier in order to provide pump redundancyxe2x80x94but at high cost.
The power output of conventional optical amplifiers has recently been increased by the introduction of pump modules containing several semiconductor lasers whose outputs are wavelength division multiplexed into a single optical fibre. Although the output power obtainable from such an optical amplifier containing one of these pump modules is sufficient for amplifying many channels simultaneously, the approach is expensive, is currently limited in powers to around 1 W, and offers limited pump redundancy.
Higher-power optical amplifiers and fibre lasers can be constructed using double-clad optical fibres containing a single-mode waveguiding core doped with rare-earth ions (such as Erbium or Erbium/Ytterbium) and a multi-mode inner cladding formed by the silica cladding guiding against an outer cladding with lower refractive index. This outer cladding is typically a polymer outer cladding. However, it is cumbersome to separate the signal path to the single-mode core from the path required to launch pump powers into the inner cladding. Several techniques have been tried including separating the beams with dichroic mirrors, side pumping using a multimode coupler, and etching pits into the double-clad fibre. However, none of these techniques provides a simple, effective way of reliably introducing the pump energy into the optical amplifier or fibre laserxe2x80x94especially if several pump lasers are required in order to provide pump redundancy. This issue is of concern for high-power fibre lasers where there is a requirement to introduce the pump energy from several to tens of pump diodes into the laser cavity. No effective, reliable and cost-effective way to achieve this exists in the prior art.
An associated problem is that introducing signal conditioning into the optical amplifier can be difficult. For example, it is often desirable to compensate for the spectral gain variation within the optical amplifier, or to introduce a filter to compensate for the dispersion in a telecommunication link. This requires ready access to the signal, which can be difficult for most amplifier configurations. A requirement therefore exists for an amplifier and laser design where it is simple to insert added functionality.
Today""s optical telecommunications networks are increasingly based on wavelength division multiplexingxe2x80x94the simultaneous transmission of many wavelength channels through the same fibre. As the networks expand, these wavelength channels can originate from different locations. This places stringent demands on the management of the network, especially on the performance of optical amplifiers dispersed throughout the network. The wavelength channels arriving at an optical amplifier are unlikely to have equal powers (i.e., they are unbalanced), and the power of an individual wavelength channel can be suddenly and unexpectedly increased. This unbalance and the changing of the power levels in individual channels is referred to as granularity. Prior art optical amplifiers experience problems with unbalanced wavelength channels in that the highest power wavelength channel can be amplified more than the other channels, thus increasing the unbalance. In addition, the sudden changing of the power level in one wavelength channel can cause instabilities in the optical amplifier that carry over to other channels. One of the most robust solutions to remove the granularity is to separate all the wavelength channels prior to amplification, amplify the channels, and then recombine the channels for retransmission. The major problem with this approach is that networks can transmit over one hundred wavelength channels through a single optical fibre. The cost of prior-art optical amplifiers makes this solution unattractive.
The cost issue of optical amplifiers is also a problem as the networks expand into metropolitan areas, the expansion being driven by the insatiable demand for bandwidth for internet, data, mobile phones and cable television. Prior art optical amplifiers are too expensive and this is currently limiting the expansion of the networks.
Erbium-doped fibre amplifiers have revolutionized optical telecommunications over the last ten years. They are finding more and more uses, for instance for compensation of switching losses. The increasing need for capacity in telecommunication networks drives not only amplification requirements, e.g., output power and gain flatness for wavelength division multiplexing applications, but also the required number of amplifiers in a system. Erbium doped fibre amplifiers have remained xe2x80x9cstand-alonexe2x80x9d devices, with individual amplifiers separately packaged. Component count as well as cost then holds back penetration of the optical amplifiers into different application areas that require a large number of amplifiers at a low cost. Instead, the drive has been towards purpose-built optical amplifiers with high specifications (bandwidth and output power) for use in applications that can tolerate a high cost.
It is therefore an aim of the present invention to obviate or reduce the above mentioned problems.
According to a non-limiting embodiment of the present invention, there is provided an optical fibre arrangement comprising at least two optical fibre sections, the optical fibre sections each having an outside longitudinally extending surface, and the outside longitudinally extending surfaces being in optical contact with each other.
The invention further includes an optical amplifier constructed from such an optical fibre arrangement, and especially a parallel optical amplifier with multiple amplifying fibres. This embodiment of the invention realizes particular commercial application in optical telecommunication networks.
The apparatus and methods of the invention can enable pump power to be conveniently coupled into optical amplifiers and lasers.
The apparatus and methods of the invention can enable optical amplifiers and lasers to be constructed that are more immune to pump failure than are prior art devices.
The apparatus and methods of the invention can enable optical amplifiers and lasers to be conveniently constructed having additional functionality.
The apparatus and methods of the invention can enable a route for lower cost optical amplification particularly useful in optical networks.
The apparatus and methods of the invention can reduce the effects of granularity in optical networks.
The apparatus and methods of the invention can enable individual wavelength channels in WDM networks to be amplified and balanced.
The apparatus and methods of the invention can enable high-power optical amplifiers and high-power fibre lasers to be constructed.
The invention also provides an optical fibre arrangement comprising a plurality of optical fibres each having an outside surface and defining a length, and wherein the outside surface of at least two adjacent optical fibres are in optical contact along at least a respective portion of their lengths.
The optical fibre arrangement can comprise a plurality of optical fibres that are surrounded by a coating material along the length of the optical fibre arrangement.
The invention also provides a method for manufacturing an optical fibre arrangement comprising the following steps: providing a plurality of optical fibre preforms, each optical fibre preform comprising a plurality of optical fibres, each optical fibre defining an outside surface and a length, mounting the plurality of optical fibre preforms in a fibre drawing tower, drawing a plurality of optical fibre from the plurality of optical fibre preforms under a drawing tension and at a drawing speed, the drawing tension and the drawing speed being selected such that the outside surface of at least two adjacent optical fibres are in optical contact along at least a respective portion of their lengths. The plurality of optical fibres can be twisted or intertwined during the drawing process.
The plurality of optical fibres can be coated by passing the fibres through a coating cup filled with a coating material.
The invention also provides a method for manufacturing an optical fibre arrangement comprising the following steps: providing a plurality of optical fibres, each optical fibre defining an outside surface and a length, pulling the plurality of optical fibre under a drawing tension and at a drawing speed, the drawing tension and the drawing speed being selected such that the outside surface of at least two adjacent optical fibres are in optical contact along at least a respective portion of their lengths. The plurality of optical fibres can may be twisted during the drawing process.
The invention also provides an amplifying optical device having an optical pump and an optical fibre arrangement comprising a plurality of lengths of at least one optical fibre, each length of the optical fibre defining a longitudinally extending outside surface, the arrangement being such that the outside surfaces of at least two adjacent lengths of the optical fibre are in optical contact with each other.
The amplifying optical device can be an amplifier comprising a plurality of amplifying fibres, each having an input and an output, at least one pump optical fibre having two ends, and a pump that supplies pump energy connected to the pump optical fibre, the amplifier being configured such that the pump energy is shared by the plurality of amplifying fibres.
The amplifying optical device can be an amplifier comprising at least one input fibre, a first multiplexer connected to the input fibre, a coupler, and at least one output port connected to the coupler, the amplifier being configured such that at least one of the amplifying optical fibres is connected to the first multiplexer and at least one of the amplifying optical fibres is connected to the coupler.
The fiber arrangement can serve to couple light from one or more pump fibers into one or more signal fibers. The signal fibers can incorporate a core for guiding signal (or generated in case of a laser) light through the arrangement. The cores can be single-moded. The region surrounding the cores of these fibers is capable of guiding pump light. These signal fibers each have two ends, at least one of which is accessible, for example in the sense that other fibers can be spliced to said signal fibres. The pump fibers are capable of guiding highly multi-moded pump beams from a pump source into the arrangement. The pump fibers each have two ends. Light can be launched into the pump fibers through their ends.
The invention also provides an amplifying arrangement comprising a plurality of optical amplifiers each having a plurality of amplifying optical fibres and further comprising a second multiplexer connected to each first multiplexer.
The amplifying optical arrangement can comprise a plurality of optical amplifiers and an optical device, the amplifying optical arrangement being configured such that the optical device is connected to at least one optical amplifier.
The optical device can comprise at least one of an optical router, an optical switch, a gain flattening filter, a polarizer, an isolator, a circulator, a grating, an optical fibre Bragg grating, a long-period grating, an acousto-optic modulator, an acousto-optic tunable filter, an optical filter, a Kerr cell, a Pockels cell, a dispersive element, a non-linear dispersive element, an optical switch, a phase modulator, a Lithium Niobate modulator, or an optical crystal.
The invention also provides an amplifying optical device comprising a fibre arrangement formed as a coil of a plurality of turns of amplifying optical fibre, the fibre comprising an inner core and an outer cladding, the arrangement being such that the claddings of adjacent fibres of at least a pair of the turns touch one another. The coil can be coated, and the coil can comprise at least one amplifying optical fibre and at least one pump optical fibre.
An amplifying optical device such as a laser or an optical amplifier constructed from a coil of uncoated optical fibre has the following advantages compared to the prior art:
1. It is based on an all-glass amplifying fibre: This overcomes the power limitations associated with polymer outer claddings.
2. It has a single pump-guiding cladding with an embedded core, but no coating or outer cladding. This overcomes the problems with accessing the pump waveguide for side-splicing that arises in other all-glass structures.
3. It uses a glass-air waveguide for guiding the pump. This results in a high NA greater than 1.
4. It has a substantially reduced glass-air surface area compared to previous fibre laser designs. This reduces the losses that arise at such an interface.
5. It can be formed by coiling a fibre and fixing it (e.g., by fusing) into a rigid body.
6. It eliminates the requirement (in a prior-art cladding-pumped fibre amplifier or fibre laser) that the fibre from which this new structure is made must be able to guide the pump i.e., is large enough and with sufficient NA (and assuming that the amplifier or laser structure is made by coiling a fibre). Thus, a fibre with a much smaller outer diameter and hence a much lower passive-cladding to active-core volume ratio (=area ratio) can be used. This improves pump absorption and thereby efficiency. Instead, it is enough that the structure as a whole can guide the pump. For instance, a pump coupler can be side-spliced to the structure rather than to a single point on a single fibre or the pump energy can be introduced with a plurality of pump optical fibres that can be of such a size that they can be located in the interstitial spaces within the coil.
7. It eliminates the need for special geometries for improved pump absorption. In a prior-art cladding-pumped fibre, special measures like off-center cores, non-circular claddings, or bending of the fibre to special geometries is normally needed to improve the pump absorption because some pump modes are otherwise absorbed too slowly. The disclosed structure can help to eliminate the geometrical similarities and symmetries between the pump waveguide and signal waveguide (core). This improves the pump absorption even in the absence of any further measures as described above.
8. It can be securely supported at a few points in space. This reduces any excess pump propagation loss that can arise at such supporting points, because these perturb the waveguiding air-glass interface. The structure can be supported by the fiber pig-tails. These are typically coated, and so can be fixed without any pump loss.
9. It provides means for preventing pump light from leaking out through pump delivery fibres.
The invention also provides an optical fibre laser comprising an amplifying optical device comprising a pump source and an optical fibre arrangement, and an optical feedback arrangement for promoting light generation within the laser.
The invention also provides a method for reducing the granularity in optical telecommunications network, which method comprises providing at least one amplifying optical arrangements as described herein having a plurality of amplifying optical fibres in at least one location within the network.
The invention also provides an optical telecommunications network comprising at least one of the amplifying optical arrangements described herein, and having a plurality of amplifying optical fibres.